exporting eyes with gltf to unity
glTF 2.0 has a specific PBR material model, for transfer to realtime rendering engines.  It cannot support arbitrary arrangements of Blender nodes, as for example a mobile phone wouldn't be able to handle realtime rendering of complex Blender nodes.  Instead, the glTF material definition has a strict number of fields, corresponding to a particular interpretation of physically-based materials.

The Blender Manual documents how to export materials to glTF.